drm/i915: Don't read sprite LP2+ registers on ILK/SNB

Sprite LP2+ registers don't exist on ILK/SNB so don't read them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>